Remove unused code
Rustc has a builtin lint for detecting unused code inside a crate, but when an item is marked `pub`, the code, even if unused inside the entire workspace, is never marked as such. Therefore, I've built [warnalyzer](https://github.com/est31/warnalyzer) to detect unused items in a cross-crate setting.
Closes https://github.com/est31/warnalyzer/issues/2
Codegen backend interface refactor
This moves several things away from the codegen backend to rustc_interface. There are a few behavioral changes where previously the incremental cache (incorrectly) wouldn't get finalized, but now it does. See the individual commit messages.